A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. One type of lithographic apparatus is a scanner, in which each target portion is exposed by scanning a pattern reticle through a projection beam in a given direction (the “scanning” direction), while synchronously scanning the wafer substrate parallel or anti-parallel to this direction. Successful scanning requires extremely precise synchronization between the moving reticle and wafer stages during the exposure. For example, the scanning speed of a scanner is a key factor affecting quality and throughput.
During processing, some critical layers may require lithography steps to expose a dummy image near wafer edges. The dummy image exposures can account for 20% or more of the total layer exposure time. Generally, a maximum scanning speed is selected for the exposure of a single layer. Moreover, a maximum scanning speed is often selected for use with both critical and non-critical layers.